1) I do not have a preference between Cobra or Ultra Blazer if the pigweed plants are 2″-4″ tall.
2) I do prefer Cobra over Ultra Blazer when plants get bigger than that. But, I would not expect any miracles when plants get larger than 4″. For the record, labeled maximum heights for Ultra Blazer (24 oz/A) and Cobra (12.5 oz/A) are 4″ tall/6 leaf and 3″ tall/6 leaf, respectively.
3) If plants are too big, grower is wasting his time and money on a revenge spray.
4) Other options for controlling large pigweed include hand-weeding, mechanical cultivation, and/or using paraquat in a non-selective applicator (wiper, rope-wick, etc.).